Cellular networks are constantly evolving. For example, fifth generation (5G) networks are evolving from fourth generation (4G) Long Term Evolution (LTE) networks, which, in turn, evolved from third generation (3G), and second generation (2G) networks. Because the evolution of these technologies takes time, today's cellular networks are “heterogeneous” by employing a mixture of newer (e.g., 5G) and legacy (e.g., 4G/3G/2G) radio access technology (RAT) systems. This heterogeneous quality of cellular networks is likely to exist in future cellular networks as well. Even though a user equipment (UE) may employ the latest cellular radio technology, the UE often continues to support legacy RAT systems for various reasons.
Upon 5G's official deployment, 5G-compliant UEs will prefer to use a 5G RAT system and core network to establish communication sessions. This is because 5G technology promises to offer relatively high capacity, reliability, and data throughput as compared to the available legacy RATs. In most UEs, a choice of which RAT system to employ depends primarily on which RATs are available to the UE at its present geographic location. Nevertheless, when a 5G RAT is available, and a communication session (e.g., a voice call) is established using a 5G RAT, it is still possible for something to go wrong. For instance, an issue with the telecommunications network itself (e.g., the data core, the IP Multimedia Subsystem (IMS) core, etc.) may render a 5G-compliant UE unable to establish a session using the 5G RAT.
In 5G, however, fallback procedures are largely non-existent due to the fact that the 5G core network is not backwards compatible with existing 4G technology. Moreover, Circuit Switched Fallback (CSFB) procedures are not supported by the 5G network. If a traditional fallback approach is adopted for 5G-compliant UEs—say one that is similar to the approach in CSFB—one might expect such a programmed UE to always retry a failed 5G communication session using an available legacy RAT(s), if one is available. In this approach, if repeated fallback attempts are unsuccessful, computing resources (e.g., processing resources, power resources, network bandwidth resources, etc.) may be wasted trying, but failing, to reattempt a communication session on a legacy RAT. The impact of this approach can be severe, especially considering the vast number of UEs that may be simultaneously retrying to establish respective 5G communication sessions that failed (e.g., when a widespread outage occurs). In addition to the unneeded resource consumption on the UE, this can place significant stress on a cellular network, especially when capacity for legacy sessions dwindles in the future due to an anticipated reduction in paid licenses, for example.